FIX Treat value not in SingleSelectField options as blank #2096

Job	Run time
CI / Generate matrix	12s
CI / Context	1s
CI / Check governance	1s
CI / 8.1 prf-low mysql57 phpunit framework-core	5m 44s
CI / 8.2 mariadb phpunit framework-core	5m 52s
CI / 8.3 mysql80 phpunit framework-core	7m 8s
CI / 8.3 mysql84 phpunit framework-core	6m 40s
CI / 8.1 prf-low mysql57 phpunit framework-orm	10m 30s
CI / 8.2 mariadb phpunit framework-orm	10m 7s
CI / 8.3 mysql80 phpunit framework-orm	11m 34s
CI / 8.3 mysql84 phpunit framework-orm	11m 33s
CI / 8.1 prf-low mysql57 phpunit cms	2m 47s
CI / 8.2 mariadb phpunit cms	1m 53s
CI / 8.3 mysql80 phpunit cms	1m 39s
CI / 8.3 mysql84 phpunit cms	1m 39s
CI / 8.1 mysql57 phplinting	2m 33s
CI / Dispatch tag patch release	0s
	1h 19m 53s

Provide feedback